The present invention relates to electrochemical cells; and more particularly, it pertains to an improved electrode for use in an electrochemical cell in the field of voltammetry.
Broadly speaking, electrochemical cells include a reference electrode and an indicator or active electrode immersed in an electrolyte, together with some means for measuring the voltage between the electrodes and the current flowing through the electrodes. If a separate, external source of current or voltage is provided to energize the electrodes, the cell is sometimes referred to as a forced cell.
A passive cell is one in which no external source of voltage or current is applied to the electrodes. An example of this type of cell is an ion-selective electrode and reference electrode used to measure hydrogen in activity.
Polarographic measuring systems apply a potential to a forced cell which operates on the principle that when a varying electromotive force is applied to the electrodes of the cell, the resulting current is representative of the concentration of a trace material (being reduced) in the electrolyte. One electrode is energized with a positive polarity and it is resistant to changes. The non-polarizable electrode exhibits a current which is representative of the reduction process taking place. There is an exponentially increasing current at a particular voltage which is characteristic of (and hence identifies) the particular material being investigated.
In polarography, probably the most commonly used type of electrode is called a dropping mercury electrode. It includes a length of capillary tubing, such as a marine barometer through which pure metallic mercury flows. As mercury flows through the tube, drops form at the tip of the capillary. As each drop grows to a certain size, it falls off and another drop begins to form. The dropping mercury electrode as well as a reference are placed into the solution under study. Other systems use two working electrodes. The cell voltage is then applied to them, and a reference electrode which is not biased is used to directly measure the cell voltage.
Typically, the capillary tubing will be fed by a larger source of mercury into which a metallic electrical contact, such as a platinum wire, may be provided to complete the circuit.
As each drop forms, the area increases continuously, thus increasing the current flow until the drop falls, at which time the current decreases to zero, and the cycle repeats itself. The currents involved are normally so small (of the order of microamps) and the voltage so nominal (of the order of one-two volts) that the iR drop across the electrolyte may be ignored when compared with the voltage at the working electrode. The current oscillates between a maximum and a minimum value so as to trace out the particular voltage-current characteristic for the material under study.
As already mentioned, a typical voltage-current characteristic for the oxidation or reduction of a substance starts out linearly and then increases exponentially at a particular applied potential characteristic of the substance. The curve then levels off to form a plateau. Reduction occurs only in the narrow layer surrounding the electrode, and the leveling off is caused by the ability of the species to diffuse through this layer and to come into contact with the electrode surface. Hence, the plateau is sometimes referred to as the diffusion plateau, after Hevorsky, the inventor of polarography, as described by Meites, et al in Advanced Analytical Chemistry, McGraw-Hill (1958).
This type of polarographic cell employs platinum or gold as the indicator electrodes, and this type of electrode may be agitated to limit the sensed current to diffusion of the reducible substance to the electrode, thereby increasing the diffusion current in relation to the polarizing current. The diffusion current also depends on the size of the electrode, the temperature of the solution, the concentration of diffusing material, the electrode material, properties of the solution/electrode film, etc.
Another system used a disc and rotating ring for electrodes, see Johnson, et al, Analytical Chemistry, Vol. 40, No. 3 (1968). Other methods of voltammetry are described in Nicholson, et al, Analytical Chemistry, Vol. 37, No. 2 (1965).
In practice, the equilibrium of the electronic exchange electrode-solution is not achieved instantaneously, requiring that the potential applied to the electrodes be raised to a higher value than that theoretically expected. This difference or "lag" with respect to the reversible value is referred to as "overvoltage". Overvoltage increases with current, and it results in a hysteresis effect relative to the reversible value of voltage. For example, it is known that the overvoltage of H.sup.+ on mercury is about 600 mv. whereas discharge of H.sub.2 occurs as expected when polished platinum is used as the indicator electrode. A dropping mercury electrode has certain inherent limitations, one of which is that metallic mercury is oxidized fairly readily. Thus, even in solutions containing only non-complexing anions, such as perchlorate and nitrate, the oxidation of mercury begins at a potential near +0.4 volts. In such a system, the concentration of dissolved mercurous or mercuric ions present around the drop will be quite large even at potentials onyl very slightly more positive than +0.4 volts. This gives rise to a large negative current, rendering it impossible to measure or even detect a small additional current due to the reduction or oxidation of the substance under study. This limit of the range of potentials within which useful data can be obtained is even more restrictive in solutions containing ions like chloride, cyanide and hydroxide, which combine with either mercurous or mercuric ion and thus facilitate the oxidation of metallic mercury. Another disadvantage in the use of mercury as an electrode is that it readily alloys with heavy metals so that the process may not be reversible under conditions of long polarization.
The present invention provides for an electrode in an electrochemical cell, such as are used in polarography, which is formed from tantalum and carbon. Tantalum is formed into the desired shape of the electrode, and it is then placed in a vacuum furnace in the presence of carbon and heated to 800.degree.-1000.degree.C. until the surface of the tantalum metal assumes a gold color. The resulting composition is the electrode material which I have found to have, surprisingly, an extreme lack of re-activity. For example, whereas tantalum is dissolved by hydroflouric acid, this material is not affected by it. Further, whereas carbon is visibly attacked by aqua regia, this material is not attacked by aqua regia. In addition, this material is highly resistant to electrochemical corrosion. These characteristics are advantageous to any electrode in an electrochemical cell, and it is therefore believed that the material of the present invention has application as an electrode in electrochemical cells broader than that polarography, although most of the results discussed hereinafter are based upon polarographic studies.
The material of the present invention has advantages over platinum as used as an electrode in that it is much more economical than platinum and it is not oxidized or dissolved in HF or aqua regia. When a positive potential is applied to platinum, it becomes oxidized, and if the electrode is then scanned negatively, a hysteresis effect of the type already mentioned is exhibited. Platinum also forms alloys with heavy metals. The material of the present invention has advantages over mercury as an electrode material because it does not alloy with the heavy metals. This is a particularly important characteristic in toxicological studies, particularly under prevailing circumstances wherein the heavy metals are among the most prominent poisons found in tissue.
The present invention also permits forming electrodes of various shapes by forming the tantalum substrate first (while in its metallic form) and then coating with carbon. When scanned in a polarographic cell, an electrode of the tantalum/carbon material exhibits no evidence of hysteresis, thereby resulting in a sharper, more highly sloped voltage-current characteristic for a particular substance being studied. It appears that this particular property also indicates that there is no oxidation of the material or any other film formation on the surface of the material since it is believed that oxidation is a principal cause of the overvoltage and hysteresis phenomena mentioned above for platinum and other metals. Because of the physical form of the tantalum prior to firing in the vacuum furnace, the resulting electrode can be made to have very high surface area, and this permits the detection of substances at levels much lower than had been possible with conventional dropping mercury electrodes or with platinum electrodes.
The advantageous properties of the present invention would also be useful in electrolytic cells, for example, in production-size commercial cells for the production of chlorine by electrolysis or in other such cells. Normally, the range of voltages employed will be limited by the nature of the electrolyte rather than any voltage limitation on the carbon/tantalum material.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of preferred embodiments accompanied by the attached drawing.